Robin Olsson

A safe design of composite tanks for liquid hydrogen is one of the strongest challenges in the coming years. This paper describes an approach to characterize the influence of cryogenic temperature on the mechanical properties of a thin-ply carbon/epoxy material manufactured by wet filament winding. The aim is to obtain the properties required for lightweight design of a suitable thin-ply layup for future use as a cylindrical tube in a linerless tank for storage of liquid hydrogen at cryogenic temperature and interior pressure. A methodology is presented for determining the ply properties required as input parameters in analytical models describing the constitutive relationships, strength-based failure criteria, and fracture mechanics for in-situ strength predictions within thin-ply theory. Therefore, tensile and shear behavior, as well as interlaminar toughness of flat wet filament wound laminates have been determined experimentally at room and cryogenic temperature. Additionally, the variation of coefficients of thermal expansion and dynamic mechanical analyses over temperature have been investigated. Since the ply properties are a function of the fiber volume fraction and quality obtained during wet filament winding, an integration of micromechanics is provided on how to adjust properties based on different fiber volume fractions obtained from nominally unidirectional coupon specimens for use in design of real angle-ply cylinders. A methodology is proposed to reach the relevant mechanical properties for design making use of a limited set of experimental properties, based on challenges to test at −253°C (20 K). An outlook on potential improvements regarding mechanical testing at cryogenic temperature is provided.

This paper describes design, manufacture, and testing of a linerless composite vessel for liquid hydrogen, having 0.3 m diameter and 0.9 m length. The vessel consists of a composite cylinder manufactured by wet filament winding of thin-ply composite bands, bonded to titanium end caps produced by additive manufacturing. The aim was to demonstrate the linerless design concept with a thin-ply composite for the cylinder. The investigation is limited to the internal pressure vessel, while real cryogenic tanks also involve an outer vessel containing vacuum for thermal insulation. Thermal stresses dominate during normal operation (4 bar) and the layup was selected for equal hoop strains in the composite cylinder and end caps during filling with liquid hydrogen. Two vessels were tested in 20 cycles, by filling and emptying with liquid nitrogen to 4 bar, without signs of damage or leakage. Subsequently, one vessel was tested until burst at almost 30 bar.

A novel intralaminar model has, for the first time, been applied and validated for the rate-dependent failure of multidirectional carbon/epoxy laminates. Quasi-static compressive failure is evaluated by the growth of intralaminar rate-dependent damage combined with the interaction of cohesive zones for interlaminar delamination. A special feature of the intralaminar model is the homogenised ply response, allowing simultaneous damage-degradation of the polymer matrix combined with the fibres. To model the observed quasi-brittle failure response of the plies under finite deformation, we have used a viscoelastic-viscoplastic matrix combined with damage and isotropic hardening behaviour. Elastic transverse isotropy is used to model the fibre reinforcement of the plies. Standard cohesive surfaces are used to model the initiation and propagation of delamination. Numerical simulations using ABAQUS/Explicit are performed to predict the growth and delamination of intralaminar damage under compression in different laminates with 56 plies of IM7/8552 carbon/epoxy. Predictions of stress versus strain and damage growth are shown to agree well with experimental results for a range of strain rates and stacking sequences.

This paper details a complete crush model for composite materials with focus on shear dominated crushing under a three-dimensional stress state. The damage evolution laws and final failure strain conditions are based on data extracted from shear experiments. The main advantages of the current model include the following: no need to measure the fracture toughness in shear and transverse compression, mesh objectivity without the need for a regular mesh and finite element characteristic length, a pressure dependency of the nonlinear shear response, accounting for load reversal and some orthotropic effects (making the model suitable for noncrimp fabric composites). The model is validated against a range of relevant experiments, namely a through-the-thickness compression specimen and a flat crush coupon with the fibres oriented at 45° and 90° to the load. Damage growth mechanisms, orientation of the fracture plane, nonlinear evolution of Poisson's ratio and energy absorption are accurately predicted.